lend-lease

WWII started in 1939 in Europe, and by 1940 the United Kingdom was in a fight for its life. While many Americans sympathized with the British and the (quickly-defeated) French, having just fought with them 22 years before in World War I, isolationism was alive and well in the U.S., and President Franklin D. Roosevelt was unwilling or unable to take the risk of an overt "entangling alliance" in the 1940 election year. He did, however, exchange some obsolete destroyers for a 99-year lease on four British bases in the Western Hemisphere.

Once the election was over, FDR essentially had a free hand to offer whatever assistance possible short of committing U.S. troops, and was able to push the Lend-Lease Act through Congress on 22 March 1941. This allowed him to send ships, armor, and materiel to pretty much any country in the anti-German alliance, with the implication that the items were only being let -- compensation was expected sooner or later. The UK and the Soviet Union were the two main recipients of lend-lease aid; the UK received about $30 billion and the USSR about $11 billion of the $48 billion total. After the war, various settlements were made reducing the load on debtor countries; the UK's debt in particular was reduced to $5.2 billion, an amount that will be fully paid off in 2005.

Lend-lease became a thing of the past shortly after the 7 December 1941 Japanese attack on Pearl Harbor. When the U.S. declared war on Japan, the remaining Axis members, Germany and Italy, responded in kind by declaring war directly on the U.S. This allowed FDR to start directly aiding the Allies, both with supplies and men.

Sources: http://uboat.net/allies/documents/lend-lease.htm, http://answers.google.com/answers/threadview?id=372591